1. Field of the Invention
The present invention relates to a laser annealing method and apparatus that anneal the surface of an object to be irradiated by laser irradiation.
2. Description of the Related Art
In recent years, a high-performance thin film transistor has been fabricated by the laser annealing to an amorphous semiconductor film or a crystalline semiconductor film that is formed on an insulated substrate such as a glass substrate. A glass substrate is inexpensive and excellent in terms of machining property as compared to a quartz substrate that has been commonly used in the past. For this reason, the glass substrate has an advantage of being formed to have a large area. The reason to use laser in crystallizing a semiconductor film is that a melting point of a glass substrate is 600° C. or less but it is possible to melt and crystallize a semiconductor film without heating the glass substrate if using laser.
Since a crystalline silicon film formed by using laser annealing having high mobility, a thin film transistors for driving pixels and a TFT for a drive circuit may be fabricated on one glass substrate. The crystalline silicon TFTs are used in a liquid crystal display of a portable or digital still camera, and has been widely provided as a product.
Currently, a laser annealing apparatus shapes a pulsed laser beam, which is generated from a source of laser such as excimer laser or solid-state laser, into a linear beam that has a profile of, for example, 100 to 400 mm×0.05 to 0.5 mm; irradiates the amorphous silicon film, which is deposited on the glass substrate, with the linear beam; and melts and solidifies the silicon film in order to crystallize the silicon film. If the glass substrate is transferred, it is possible to irradiate the entire surface of the silicon film deposited on the glass substrate which is the size of, for example, 730×920 mm from amorphous silicon to crystalline silicon.
A laser annealing method using the linear beam has some problems, and a particularly serious problem is that visible stripes appear in a direction perpendicular to a long axial direction of the linear beam and in a direction parallel to the long axial direction. Hereinafter, stripes perpendicular to the long axial direction of the linear beam are called as “vertical stripes”, and stripes parallel to the long axial direction are called as “horizontal stripes”. Further, the vertical stripes and the horizontal stripes are collectively called as “irradiated stripes”.
FIGS. 1a and 1b of the following Patent Document show a state where the irradiated stripes appear on the film. If an active matrix type liquid crystal display or organic EL display is manufactured using the film having the irradiated stripes, there is a disadvantage in that the same stripes as the irradiated stripes appear on a picture of the display. In particular, since the organic EL display is more sensitive to the irradiated stripes than the liquid crystal display, there is a demand for a countermeasure that is effective to reduce the irradiated stripes.
Some methods of reducing irradiated stripes are proposed in the following Patent Documents 1 to 4. The uniformity of the beam, interference, the deviation of laser shots, the change of the profile of laser, and the like have been mentioned as factors generating the irradiated stripes, but have not been clarified.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 10-294288
[Patent Document 2]
Japanese Unexamined Patent Application Publication No. 2001-68430
[Patent Document 3]
Japanese Unexamined Patent Application Publication No. 10-242073
[Patent Document 4]
Japanese Examined Patent Application Publication No. 5-41006
The present inventors came to specify factors generating the irradiated stripes after a process of trial and error for clarifying the cause of the irradiated stripes.
First, the present inventors investigated the pattern of vertical stripes when the position of a lens array was shifted by 1 mm in a direction corresponding to a long axial direction of a linear beam in a homogenizer for homogenizing the light intensity in the long axial direction of the linear beam. As a result, the pattern of the vertical stripes was changed. The reason for the change was considered as follows:
(a) the vertical stripes appeared depending on the profile of a laser beam emitted from a laser source, or
(b) the vertical stripes appeared due to scattered light generated on the surface of a lens of the homogenizer.
Then, in order to determine which is right, the present inventors made two kinds of laser beams enter the same homogenizer optical system, and investigated vertical stripes. In general, the light intensity profile of laser is an intrinsic property of an each laser oscillator. Therefore, if the pattern of the vertical stripes is different, it may be determined that the irradiated stripes depend on the profile of a laser beam. However, as a result of the investigation, the same pattern of the vertical stripes appeared. From this result, it was determined that the irradiated stripes were caused by the scattered light generated on the surface of the lens. That is, it could be concluded as follows: since a laser beam was scattered on the surface of the leans and the uniformity of the laser beam thus deteriorated on the irradiated surface, the irradiated stripes appeared.
In general, the surface accuracy of a lens is λ/4 or λ/10. However, a lens used in laser annealing, particularly, a lens provided on the rear side is a large-sized lens having a length of 100 mm or more. Accordingly, since it is difficult to work the lens and to measure the surface accuracy of the lens, the surface accuracy of the lens significantly deteriorates and may be λ or more. According to the current technology, since there is a limit to the machining accuracy of a large-sized lens, it is difficult to further improve the surface accuracy. For this reason, other countermeasures should be taken to improve the surface accuracy.